"They seem very ferocious," Jana Goyens, from the University of Antwerp, who led the study, told BBC News.

"But long [jaws] should not be very efficient when it comes to transferring very large bite forces - it would seem from a mechanical point of view that they would not bite forcefully."

This is because the force generated by muscles in the beetle's head has to be transferred down to the end of each jaw, or mandible. So, just like a very long lever, the force has to work over a long distance.

You can experience this rule of mechanics with your own body.

If you hold your arms straight out in front of you and press your hands together, it is difficult to produce a great deal of force. But if you bring your hands in close to your chest, you can push your palms together with much more force.

Beetle battles

Ms Goyens staged and filmed fights between the male beetles to determine the distance between their jaws when they grab an opponent.

In detail: Stag beetle

Scientific name of stag beetle family: Lucanidae

The insects' enlarged jaws resemble stags antlers

Adults do not eat and live for only a few weeks as beetles, with the rest of their life - four to six years - spent underground as developing larvae

Distribution of UK species (Lucanus cervus) is primarily in the south and south-east of England, but numbers have fallen sharply in the past four decades

She then measured the bite force that the beetles could produce at the end of their jaws, and studied scans of the internal anatomy of their heads.

These measurements revealed first that the male head is a lot wider than the female.

"That makes space for longer input levers," Ms Goyens said. Like the handle of a pair of pliers, there is a long lever inside the beetle's head that is hinged to each of its jaws.

"And that enhances the force," she said.

"Second, their entire head is filled with these muscles to close the jaws.

"The head shape of the males is adapted to make space for these enormous muscles. And of course, the bigger the muscles, the larger the muscle force."

Ms Goyens said this was an example of just how extreme evolutionary changes could be.

"Sexual selection has had a very large impact on their anatomy," said Ms Goyens. "To maintain their jaws as a useful weapon, they had to change their entire head."